Fact: men have more sex than women: If you believe that, you've probably been reading a survey. What do they really tell us? Angela Neustatter reports

SCIENTISTS have gone to extraordinary lengths to bring us insights into the sexual activities of the human race. Without their dedication, how could we have known that most Americans had no idea the average size of a penis is 5 to 7in; that women overcome with ennui when making love resort to fantasy, and that they are far more likely to be turned on by a small, well-clad pair of male buttocks than by bulging muscles.

Attempts to quantify human sexuality go back over a long time but in the past couple of decades they have become one of the most pervasive forms of conventional wisdom, so that every other week a tabloid or magazine brings out a shock-horror survey finding on our coital mores. These may be entertaining money-spinners but, according to Ruth Grigg, at the Family Planning Association, they also influence attitudes and give many people their information on health and sex matters.

There is, of course, a great difference between 'pop' surveys and studies such as The Wellcome Trust's National Survey of Sexual Attitudes and Lifestyles, published today, which is based on a large cross-section of the population, was conducted with personal interviews, and is designed to provide data for public health policies.

But do people tell the truth when stopped on the high street and asked whether they prefer fellatio to fried eggs? Do they tell it as it is when visited by such doughty researchers as in the National Survey? Kaye Wellings, a sociologist who was part of the team, acknowledges the problem: 'You cannot observe sex, in the normal private context that it occurs. There's very little sex behaviour you can check.'

Paul Brown, a sex therapist who believes that surveys, when rigorously conducted, are 'the best reality we can get', nevertheless knows how easily people deceive themselves, or tweak the truth a little. Men in surveys notoriously exaggerate their prowess and how often they have sex. In a Gallup poll conducted as a pilot for the National Survey, men, it appeared, were having 20 per cent more sex than women, who tend to veer the other way when answering questions.

Those who enjoy copulation in football shirts and 'deviant' practices may well be unwilling to say so; and as Mr Brown points out, sexual dysfunction rarely shows up in surveys. Conversely, those who believe everyone else is out there having multiple orgasms may pep up their revelations.

Another problem is simple ignorance. Dr Riley says a great many people do not understand even ordinary sexual terminology; one interviewee did not know what a womb was. The FPA finds the same, says Ms Grigg, adding: 'People phone our hotline with the most basic questions and worries. The need for decent sex education in this country is very clear, and we hope that the Department of Health will use the findings from this large survey.'

But the pursuit of sexual enlightenment has not been universally welcomed, as Celia Lowenstein explains in a Horizon film she has directed, 'The Truth About Sex' (on BBC 2 tonight at 8.10pm). The programme charts the progress of the National Survey and looks at earlier attempts to quantify sexual behaviour.

These include laboratory experiments which caused an outcry in America when conducted by Masters and Johnson, the sex researchers, such as the electronic monitoring of arousal levels in people given erotic literature to read; how women's bodies turn as pink as boiled lobsters when they are stimulated; and an orgasm photographed with a minuscule camera inside the vagina.

When Alfred Kinsey first published his works on male and female sexuality, which became best sellers, he endured much opprobrium. More recently federal funding was refused for a large sex survey in the United States because of moral opposition by religious groups. And Mrs Thatcher axed government funding for the National Survey, which has a specific goal of looking at how Aids and HIV might spread, because she thought it 'too intrusive'. Despite all this, a remarkable body of knowledge now exists. Most reassuringly, we learnt from Kinsey that large penises do not get better erections then small ones, and 21 years later researchers even showed that small penises work best. But researchers at La Sapienza University in Rome recently shocked Italy by revealing that women masturbate continually, are avid consumers of pornography and have lurid voyeuristic fantasies.

Does such knowledge really add to the sum total of human understanding and happiness? Mr Brown believes some surveys certainly can, because they 'normalise' sex and lift repressive taboos. It is also reassuring for those who live in silent anxiety about their inadequate sex lives to learn that the average person makes love 1 1/4 times a week. Mr Brown explains: 'Because sex is not talked about freely or comfortably as a subject, it is very hard for people to understand that you can be normal in a great many different ways.'

That said, he shares the criticism of John Leinkowitz, training manager at the Institute for the Study of Human Sexuality, of 'idiosyncratic' work such as Shere Hite's, which is based on unrepresentative samples but presented as a global truth. On the other hand he attaches great importance to the research of Annette Lawson, a sociologist who found in 1980 that nearly half of all married women have affairs, and the exploration of how such affairs can enhance and empower women's lives, just published in Dalma Heyn's The Erotic Silence of the Married Woman (Bloomsbury).

Incorporating that knowledge into work with relationships could be enormously helpful, Mr Brown says, as is work of Dr Janet Reibstein and Dr Martin Richards, whose survey of 100 couples found that people rarely leave their established partners for the person with whom they are committing adultery.

But surely the finding to be celebrated is that old one, first put forward by Kinsey, that sex is good for you and that sexually active people who do not take risks are generally happiest.